René Descartes believed that there was a logical path from “I think therefore I am,” to mind-body dualism. In more recent times, it has been said that no such path exists. Paul Churchland and Gary Hatfield each prominently make the case that the argument for dualism commits the masked man fallacy. That is, that the Cartesian argument relies on mere ignorance of the body to reach its conclusion. In this paper I show that the argument from “I think therefore I am,” to mind-body dualism does not depend on mere ignorance. It depends on reliable knowledge about what can be known and what can not be known. Descartes’ method of doubt leads to the conclusion that the body can never under any circumstance be known as the mind is known. The argument for dualism rests on that knowledge, not on ignorance. This paper reveals a viable Cartesian argument for mind-body dualism and explicates the missteps of Descartes’ present-day critics.